ETA (This may take a little review of the previous posts to make rather more clear)
I think I mised the point of your question, as well, you missed the intent of my post above.
When using the M1907 sling as a shooting aid, the shooter can leave the rear portion of the sling attached to the buttswivel, but loosened sufficiently as to not interfere w/ the front part which is doing all the work. Thus, the mid swivel serves no purpose in this case. In reality, there's NO reason to have a mid-swivel when using the M1907 as originally conceived.
Thus, the introduction of the mid-swivel would have been purely to accommodate the web sling which more or less requires the mid-swivel to be used whilst shooting the rifle. For carrying purposes, it really doesn't matter terribly. I like that the sling DOESN'T need reconfiguring or even adjusting when transitioning from shooting to carry modes, however it's not going to be easy to make it "parade ground" tight unless the sling is moved to the rear swivel.
This raises the interesting issue of whether the leather sling actually became just an option, or possibly obsolescent, when the mid-swivel was adopted.
Apparently, no official explanation was thought necessary, as the CES wasn't modified. Again, the M1907 sling may have been left in the kit as they were much harder to round up than the "bog standard" web sling. That gave the user an option- as hard as that may be to imagine!
Something else to make you go- "Hmmmm"